1. Field of the Invention
This invention relates to aliphatic polyurea elastomer systems.
2. Background Information
Polyurea elastomer systems are commonly recognized as suitable for coating materials, with sprayable polyurea elastomer systems being particularly useful when employed in this capacity. One of the shortcomings associated with aromatic polyurea elastomer systems, which generally are prepared by reacting an aromatic isocyanate with an active hydrogen component in the presence of an aromatic chain extender, is that they exhibit poor stability when exposed to ultraviolet radiation. This becomes particularly problematic when the substrate to be coated is continuously subjected to ultraviolet exposure, as is the case, for example, with a rooftop. The resulting ultraviolet degradation of the elastomer system typically is manifested by a change in color; a general loss of product integrity, such as cracking; and an adverse reduction in properties, such as tensile strength, tear strength and elongation, to name a few.
The most widely employed aromatic chain extender is diethylenetoluenediamine (DETDA). In addition to providing an ultraviolet unstable system, DETDA provides a rigid elastomer system which, because of its rigidity, has difficulty in assuming the detail or contour of the substrate to be coated. However, polyurea elastomer systems fabricated from, among other things, DETDA generally exhibit good processing characteristics. Thus, there is a recognized need for a polyurea elastomer system that exhibits ultraviolet stability and increased flexibility, i.e., less rigidity, while exhibiting good processing characteristics.
It is known to increase ultraviolet stability by using non-aromatic, i.e. aliphatic, active hydrogen components. For instance, Rowton, in "Cyanoethylated Polyoxypropylene polyamines: Polymer Formers of Unique Reactivity", Journal of Elastomers and Plastics, Vol. 9, October 1977, describes the use of cyanoethylated polyoxypropylene polyamines as the active hydrogen component in polyurea systems to provide light stable systems. But Rowton is silent with respect to employing aliphatic chain extenders.
In previous two-component aliphatic polyurea elastomer processes, low molecular weight polyoxyalkylene polyamines and cycloaliphatic diamines have been used quite successfully as chain extenders.
Previously known primary amine aliphatic chain extenders include trans-1, 4-diaminocyclohexane; 1,2-diaminocyclohexane; and 1,6-diaminohexane. These and other previously known primary amine aliphatic chain extenders work well, but because they react very rapidly with isocyanate, they are difficult to use in spray systems, inasmuch as polymerization occurs so rapidly that the polymer can be virtually unsprayable. Other previously known low molecular weight, linear primary amine chain extenders exhibit a rapid reactivity that result in poor mixing and elastomer cure. Previously known secondary amine aliphatic chain extenders, i.e. sym-dialkylethylenediamines, are too slow for practical, commercial applications. Additionally, elastomer systems prepared with previously known aliphatic chain extenders have exhibited processing characteristics notoriously inferior to those exhibited by systems fabricated from DETDA.
Several attempts have been made to develop polyurea elastomers and process using aliphatic chain extenders. Commonly assigned U.S. patent application Ser. No. 07/402,296, filed Sep. 5, 1989, now abandoned in favor of U.S. patent application Ser. No. 08/117,962, filed Sep. 7, 1993, describes aliphatic spray polyurea elastomers comprising an (A) component that includes an aliphatic isocyanate and a (B) component that includes (1) an amine-terminated polyoxyalkylene polyol, and (2) an amine-terminated aliphatic chain extender.
In addition applicant has previously discovered that the properties and processing characteristics of sprayable polyurea elastomers may be improved by using a cycloaliphatic diamine chain extender selected from the group including cis-1,4-diaminocyclohexane; isophoronediamine; m-xylylenediamine; 4,4'-methylenedicyclohexylamine; methanediamine; 1,4-diaminoethyl-cyclohexane; and alkyl-substituted derivatives thereof, as disclosed in commonly assigned U.S. Pat. No. 5,162,388.